As materials for a high-strength spring member to which a high load is applied, there have hitherto been a piano wire and a Si—Cr steel oil tempered wire for cold working, and a Si—Mn based steel wire for springs, etc. for hot working.
These high-strength materials each have a problem that the corrosion resistance is low. As a method for improving the corrosion resistance of a spring, painting, corrosion-resistant plating treatment or the like can be supposed. However, Si—Cr steels and Si—Mn steels cannot be subjected to plating treatment since it is feared that delayed fracture is generated therein. About a piano wire, chromate treatment, which is the most effective corrosion resistance treatment therefor, has been regulated on the basis of environmental problems in recent years. Thus, the treatment has been desired to be changed to an alternative treatment wherein no hexavalent chromium is used. Practically, only painting is a solution about the corrosion resistance. Drawbacks of these surface treatments are in that the treatments need to be conducted after a material is made into a spring form, and thus the treatments become batch treatments, whereby an increase in manufacturing costs cannot be avoided; and when the surface of a material is injured during the use of the material as a spring, corrosion advances from the injure as a starting point.
A solution against this problem is a method of adopting stainless steel as a spring material. According to this method, painting and surface treatment can be omitted; therefore, costs for manufacturing the whole of a spring can be controlled into a low value although material costs increase by the adoption of stainless steel. However, austenite stainless steel, which is particularly good in corrosion resistance, may not give a sufficient strength.
The problem that high-strength materials have is not limited only to corrosion resistance. The materials have a problem that the workability is low because of the high strength. When a spring that can resist a high load is designed, it is necessary to make the spring index thereof small in order to make the spring constant high. FIG. 3 are views for explaining a conventional spring index. When cross sections of a steel wire are in a complete round form, the average coil diameter D representing the distance from the central line of one of the cross sections of the steel wire to the central line of another and the diameter d of the steel wire are used to represent the spring index by D/d, as illustrated in FIG. 3(a). When cross sections of a steel wire are not in a complete round form, the value d′ obtained by subtracting the average coil diameter D from the coil outer diameter D′ is used to represent the coil index by D/d′, as illustrated in FIG. 3(b). A material of a spring is required to have a high workability when the spring index thereof is made small. Accordingly, the toughness is insufficient, and it is feared that cracking or seizure is generated.
As a prior art about the workability, suggested is a method of quenching and tempering steel in a wire form once, subjecting the steel to coiling, and again quenching and tempering the steel (see JP-A-2000-213579 (Patent Document 1), and JP-A-2003-073737 (Patent Document 2)). This is a method of using the quenching and tempering in the steel wire form as a treatment wherein importance is attached to toughness; subjecting the wire to coiling; and using a treatment wherein importance is attached to strength when the second quenching and tempering are carried out. This method certainly makes it possible to manufacture a high-strength spring in a state of a product that can resist a high load. However, costs increase by the two thermal treatments although the toughness problem is solved. Furthermore, the above-mentioned corrosion resistance cannot be improved.
In order to attain an enhancement in spring strength, a spring having a rectangular section or a spring having an elliptical section may be used in recent years (see JP-A-09-133168 (Patent Document 3), and JP-A-63-034335 (Patent Document 4)). This is because a large energy can be obtained within a restricted space since this method makes it possible to decrease the adhesion height. However, about a steel wire having such a section, a part of the inside of the section is locally worked; thus, when the wire is subjected to wire drawing work and spring work, a risk that a shortage in the toughness is generated increases.
Patent Document 1 JP-A-2000-213579
Patent Document 2 JP-A-2003-073737
Patent Document 3 JP-A-09-133168
Patent Document 4 JP-A-63-034335